The present invention relates to a method of wet etching of polyimide, in particular, a method of etching polyimide by using polyamide-imide and an etchant to form a staircase pattern on an insulation substrate.
Polyimide is a high temperature resistant, low dielectric constant polymer. It is widely used as an insulating material in, for instance, Flexible printed circuit (FPC), tape automatic bonding (TAB) and electronic packaging.
In general, the bonding between the polyimide and the conventional photoresist is relatively poor, and a center layer is provided in between them to increase the bonding. For instance, Japan Patent No. 2-299234, No. 2-194571, and Sho-63-274942 respectively disclose a layer of Si.sub.3 N.sub.4 film, a layer of SiO.sub.2, and a layer of aluminium alcoholate to be provided in between the layers of polyimide and photoresist. However, the anti-etching property of Si.sub.3 N.sub.4 and aluminium alcoholate is strong. As a result, dry etching, for example by the method of plasma, the activated ionic etching is employed, and a steeper sidewall can be obtained. In case wet etching is employed, an inclined smooth sidewall can be obtained. But, in the metallization process, the coated metal side wall of the steep sidewall may be too thin or not coated, which will produce discontinuation or short circuit. Although the inclined smooth sidewall can overcome the above drawbacks, the smooth surface has a relative poor bonding between the surface per se and the metal. In addition, the method of vapor coated or sputtered Si.sub.3 O.sub.4, SiO.sub.2 is complicated and expensive. The cost of aluminium alcoholate is high, which makes it unsuitable for manufacturing.
Japan Patent Sho-61-171132 discloses the use of polymer of polyimide or the like property as the center layer, in which the incomplete imidized polyimide is used as a removable layer and a resisting layer of anti-plasma is added, and then applied to the wet etching. Using O.sub.2 plasma on two layers of polyimide film of two different etched rates and by employing different thicknesses in designing the gradient of the sidewall so as to increase the bonding surface area and strength of metals during the metallization process are found to be unsatisfactory. However, the bonding strength of the polyimides is poor, and if the removeable layer of the polyimide is not removed, there is a possibility that this layer will peel off. If this layer is to peel off, the cost of manufacturing will increase and the manufacturing process will be complicated.
In accordance with the present invention, polyamide-imide having the property of or similar to the property of polyimide as the pseudomask layer, and by employing the different etched rates of the etchant respectively on polyimide and polyamide-imide and differences in undercutting forming viahole to form staircase pattern, the surface of contacting and bonding strength of metals during metallization will increase the quality of the product.